Chemical components of cell membranes
The chemical components of cell membranes The components cell membrane includes: • Lipid - cholesterol, phospholipid and sphingolipid • Proteins • Carbohydrate -- as glycoprotein Differences in composition among membranes (e.g. myelin vs. inner mitochondrial membrane) • Illustrate the variability of membrane structure. • This is due to the differences in function. Example: Mitochondrial inner membrane has high amounts of functional electron transport system proteins.
Plasma membrane, with fewer functions (mainly ion transport), has less protein. • Membranes with similar function (i.e. from the same organelle) are similar across species lines, but membranes with different function (i.e. from different organelles) may differ strikingly within a species.
Carbohydrates of membranes are present attached to protein or lipid as glycoprotein or glycolipid. 1. Typical sugars in glycoproteins and glycolipids include glucose, galactose, mannose, fucose and the N- acetylated sugars like Nacetylglucosamine, N-acetylgalactosamine and N-acetylneuraminic acid (sialic acid). 2. Membrane sugars seem to be involved in identification and recognition.
The amphipathic properties of the phosphoglycerides and sphingolipids are due to their structures. 1. The hydrophilic head bears electric charges contributed by the phosphate and by some of the bases. • These charges are responsible for the hydrophilicity. • Note that no lipid bears a positive charge. They are all negative or neutral. Thus membranes are negatively charged. 2. The long hydrocarbon chains of the acyl groups are hydrophobic, and tend to exclude water. 3. Phospholipids in an aqueous medium spontaneously aggregate into orderly arrays. • Micelles: orderly arrays of molecular dimensions. Note the hydrophilic heads oriented outward, and the hydrophobic acyl groups oriented inward. Micelles are important in lipid digestion; in the intestine they assist the body in assimilating lipids. • Lipid bilayers can also form. • Liposomes are structures related to micelles, but they are bilayers, with an internal compartment. Thus there are three regions associated with liposomes: -The exterior, the membrane itself and the inside. • Liposomes can be made with specific substances dissolved in the interior compartment. These may serve as modes of delivery of these substances. 4. The properties of phospholipids determine the kinds of movement they can undergo in a bilayer. • Modes of movement that maintain the hydrophilic head in contact with the aqueous surroundings and the acyl groups in the interior are permitted. • Transverse movement from side to side of the bilayer (flip-flop) is relatively slow, and is not considered to occur significantly.